I have to say that I haven't heard the HD650 for about 3 years now. I had and used a different setup back then. Between the HD600 and 650s I always liked the 600 more, but it was when I used the iFi Zen Stack that I preferred how they sounded with my HD600. But now… the HD650 with my DX3 Pro+ are amazing. I always thought I knew what my sound preference was, and now I see it more clearly. The iFi devices are great, but I realized how colored and warm-sounding they are. My preference is neutral-warm, which perfectly describes the HD650s. They’re great with a dead-neutral setup. The HD660S2 are amazing cans, They easily beat the HD660S in my opinion, but when I compare these two, it gets complicated. Things where I find them better:

• Bass: HD660S2 It hits harder and faster. A good example is Childish Gambino – Heartbeat. The 650s are not bad either, but they have worse bass performance compared to these. The S2s are clearly better suited for EDM and metal.

• Midrange: It's a draw (for me). I think they are really similar, although the vocals are a bit closer on the 650s, which is better for something like jazz music. I’ve listened to Hako Yamasaki’s music and I find them both great for that, but the HD650/600 are better with their vocal performance. Still, I find the instruments more detailed and better separated on the S2s. I can pinpoint them more easily with the S2s.

• Treble: HD650 That treble energy is just better in my opinion. It sounds relaxed, but not too dull. The HD660S2 can be a bit “dull” with certain tracks, but they’re great as well, in my opinion. Calvin Harris – Ocean is a great example. The HD650 treble is more delicious, light, and smooth-sounding. The S2’s treble I find better for tracks like Linkin Park – Pushing Me Away, it sounds “wider,” just enough to not be too dark for me.

• Details / Soundstage: HD660S2 Sounds are coming from further away with these. The HD650s don’t sound that veiled in my opinion. I think the S2s are “squeezed, but still wider.” Details are coming from further away, instruments are more separated and better placed. But with the HD650s, I don’t feel that “veil.” I feel like the details are better; they’re not too dark like the S2s.

I like them both for what they offer. I'm keeping them both. I highly recommend to listen to them and decide which are better for you.